1. Field of the Invention
This invention relates, in general, to photocopying and, more specifically, to control of cross-track registration in image forming apparatus and like devices.
2. Description of the Prior Art
In high speed electrostatographic reproduction apparatus, it is common practice to employ an elongated photoconductive belt or web adapted to record transferable images while the web is moving in a path in operative relation with various process stations. Typically, the web is supported by, and driven about, a series of rollers. With such a roller support system, there is a tendency for the moving web to shift laterally, or cross-track, with respect to such rollers. Various apparatus for correcting such lateral shifting of roller supported webs are known, such as crowned rollers, flanged rollers, servo-actuated steering rollers, or self-actuated steering rollers. Crowned rollers generally are not preferred for use with a web in an electrostatographic reproduction apparatus because they have a tendance to force the web toward the apex of such rollers, cause distortion of the web, and produce local stresses in the web, at the crown, which can damage the web. Flanged rollers generally are not preferred because they produce a concentrated loading at the edges of the web resulting in edge buckling, seam splitting, or excessive edge wear.
Electrostatographic reproduction apparatus, therefore, typically utilize servo-actuated or self-activated steering rollers. While such steering rollers generally correct the cross-track shifting of the web, the realignment tends to be at an uneven rate and inconsistent from one correction to the next correction resulting in the production of image frames which do not match or register.
A further problem of known servo-actuated or self-activated steering roller tracking systems is that they usually react to the absolute lateral position of the edge of the web, rather than to the actual lateral position of the web. Because the web edge is typically irregular, any misregistration due to other mechanisms, such as described hereinabove, is thereby compounded by the edge irregularity which increases, due to wear, during the web's operational life.
Efforts to correct the above problems have been made, as disclosed in the steering mechanism of U.S. Pat. No. 4,961,089; however, belt travel corrections still have not been perfected.